1. Field of the Invention
The invention relates to a fibreoptical sensor for measuring electrical fields or voltages, in which
a) a sensor fiber having a given length section is attached to at least one piezoelectric sensor element in such a manner that the electrical field to be measured leads to a change in length of the sensor fiber by means of the inverse piezoelectric effect, PA0 b) means for the interferometric detection of the change in length are provided and PA0 c) a piezoelectric modulator is provided which is driven in such a manner that the change in length of the sensor fiber induced by the electrical field to be measured is compensated. PA0 the piezoelectric modulator comprises at least one piezoelectric modulator element which is identical with the at least one sensor element with respect to material, geometric shape and crystal orientation, PA0 modulator element and sensor element are essentially at the same temperature and PA0 separate means for operating point control are additionally provided. PA0 a) light, which is conducted in a sensor fiber, is subjected to a phase shift due to a change in length of the sensor fiber induced by means of a piezoelectric sensor element, PA0 b) the phase shift is interferometrically detected and PA0 c) calibrated to zero by means of a piezoelectric modulator acting on the sensor fiber in accordance with a control signal, the intensity of the field or of the voltage, respectively, being determined from the control signal, which is characterized by the fact that PA0 d) modulator elements are used in the modulator which are identical with the piezoelectric sensor element with respect to material, geometric shape and crystal orientation, PA0 e) sensor and modulator elements are kept at the same temperature, and PA0 f) a separate operating point controller with a large phase swing is used for operating point control so that a dynamic range of at least +/-2.pi. is achieved.
2. Discussion of Background
The principle of the SMFI (= Single Mode all Fiber Interferometer) is of fundamental significance in the use of fibreoptical sensors in practice. An overview of the background relating to sensor elements and detectors is offered by the publication "Optical Fiber Sensor Technology", Thomas G. Giallorenzi et al., IEEE J. of Quantum Electronics, Vol. QE-18, No. 4, 4th April 1982.
A special case are the piezoelectric field sensors known from Patent Publication EP 0 361 619 Al. These have the characteristic of detecting exclusively one predetermined directional component of an arbitrary electrical field. If, in addition, several such sensors are connected in series in a suitable manner, the voltage between two points in space can be measured in the sense of a line integral over the electrical field. Such a voltage sensor is known, for example, from Patent Publication EP 0 316 635 Al.
A fibreoptical sensor of the type initially mentioned is known, for example, from the publication "Elimination of drift in single-mode optical fiber interferometer using a piezoelectrically stretched coiled fiber", D. A. Jackson et al., Appl. Optics Vol. 19, No. 17, 1st September 1980. This article shows how a SMFI with a piezoelectrically stretched fiber coil can be held at the optimum operating point. The fiber optical arrangement here corresponds to a Mach-Zehnder interferometer. Homodyne detection is preferably used as the detection arrangement. Whilst one arm of the interferometer is exposed to the signal to be measured, a compensator in the second arm ensures that the phase difference between the two arms is calibrated to zero. Having regard to a large dynamic range of the system, a cylindrical piezoelectric ceramic body is used as compensator onto which the fiber is wound.
In the background of the art, the temperature-dependence of the sensor signal represents a fundamental problem. In the operating range from -40.degree. to +70.degree. C. usually required, it results in errors within the range of typically some percent. In the case of quartz-based piezoelectric sensors, for example, the error is a combined temperature-dependence of piezocoefficients, dielectric constants and fiber interferometer, assuming that the compensator material is kept at a constant temperature.